The invention relates to a surface wave filter, hereafter referred to as OFW, which can be used in particular for RF applications.
In communication systems, signals are processed asymmetrically or symmetrically, whereby changes between the two operating modes often take place in the routing of the signal, and so a need arises for filters that, regarded at the input side or at the output side, can be operated asymmetrically/symmetrically or symmetrically/symmetrically. In addition, certain circuit designs require filters with the characteristics stated above, and in addition with different input and output impedances.
OFW filters that can be operated asymmetrically/asymmetrically, intended in particular for use in the RF range, are commercially available. The change from asymmetrical to symmetrical operating mode has up to now standardly been carried out by means of additional components, e.g., transformers. The impedances, i.e., the input and output impedances of these filters, are equal, and are matched by means of networks consisting of inductances and capacitances.
OFW filters that can be operated asymmetrically/symmetrically or symmetrically/symmetrically are also known. FIG. 1 shows a schematic representation of the design of such a filter, i.e., in the present case of a single-track filter with identical input and output impedances.
The structure of this OFW filter consists of an interdigital input transformer IDT3, centrically arranged, and of the reflectors REF1 and REF5, short-circuited in themselves, and interdigital output transformers IDT2 and IDT4 connected in parallel.
If the filter selectivity is to be increased, then, as FIG. 2 in turn schematically shows (identical elements are thereby designated with identical reference characters), the known single-track filter according to FIG. 1 can for example be connected in series with a filter of the same type. This filter also has identical input and output impedances and can be operated asymmetrically/symmetrically or symmetrically/symmetrically.